FAILURE
ANALYSIS
Touchstone has performed thousands of failure investigations for
a number of industries. The following are brief descriptions of
a few of them:
- A large, cast iron, crank shaft from an industrial air compressor
failed catastrophically. The metal was found to be within chemical
and mechanical specification, but shrinkage voids were found in
the area of fracture initiation. These casting defects were believed
to have provided a stress riser that led to the failure.
- A roll manufacturer asked Touchstone to investigate a rubber-covered
roll which failed catastrophically by breaking in half during
operation in a paper mill. After examination at Touchstone, engineers
found that the crack initiated at a notch which was created when
a cap screw, used to fasten a counter-balance weight, was welded
so that it would not loosen in service. The weld practice left
a sharp notch underneath the weld bead, and under normal bending
forces, a crack grew from the notch circumferentially around the
roll causing the failure. This failure mechanism was subsequently
confirmed by a team at the Massachusetts Institute of Technology.
- A helical transmission gear from a snorkel lift in a foundry
failed when all the teeth sheared catastrophically. The fracture
indicated severe loading, and it was determined that the unit
was under-designed for the application.
- A 25-ton, electrically insulated, crane hook failed in an aluminum
plant. The failed crane hook was brought to Touchstone for analysis.
The results of the analysis found that the crane hook was made
of an improper alloy and, in addition, had machine marks that
acted as stress risers. The fatigue crack initiated at these stress
risers and continued under cyclic loading until failure.
- A steel anode clamp used to hold the anode bar in an aluminum
reduction potline failed. Touchstone was contacted to determine
the cause of failure. An examination of the microstructure found
a dendritic structure instead of the expected equiaxed grains.
The cause was determined to be the failure to heat treat the clamp
after casting.
- A company contacted Touchstone concerning the failure of forged
steel brake cams. The failure consisted of the splines breaking
off of the shaft. Touchstone performed a failure investigation
and, using the scanning electron microscope, found a brittle fracture
surface, probably due to hydrogen embrittlement. Other brake cams
from the same lot were removed from service.
- A contractor experienced a problem with embrittled fasteners
when stair tread grips were being installed using self-drilling,
self-tapping screws. As the work progressed, the screws began
popping loose. The installation device had a clutch so that the
screws could not be overtorqued and, even though they were torqued
in properly, the screws would simply break. An examination at
Touchstone using the SEM found a brittle fracture surface. Because
they were plated screws, the failure was traced to hydrogen embrittlement
from the pickling and/or plating operation where the screws had
not been baked properly after production.
- An automobile radiator manufacturer contacted Touchstone concerning
leakage in the tube section of its radiators. After examination
via stereo-microscopy and scanning electron microscopy, the failure
was traced to edge mis-match and excessive temperature in the
tube welding operation which left shrinkage voids in the weld.
- An insurance company requested that Touchstone investigate a
complaint concerning a home swimming pool with unusual swelling
or buckling and lumps in the liner of the pool along with leakage
of water due to damage to the liner. The problem was traced to
extremely conservative trimming of the liner around pool lights
and around the cap of the pool. Eventually, this condition allowed
chlorinated pool water to seep between the vinyl liner and the
aluminum support for the pool liner. Pool water has enough chlorine
and is usually sufficiently acidic that it will react with aluminum
to form aluminum chloride, which then decomposes to aluminum oxide.
The formation of the aluminum oxide was the cause of the swelling
and subsequent damage.
- Defects in .001-gage aluminum foil were brought to Touchstone
for investigation. After examination via scanning electron microscope
(SEM), the defect was determined to be caused by foreign particles
which were rolled into the foil.
- A complaint from a hotel owner dealt with the great difficulty
experienced in removing old, worn carpet from hallways. The answer
to the problem was found in understanding how the system should
have been installed versus how the system was designed to be installed.
Commercial carpeting is supposed to be bonded to the padding with
a high strength adhesive, while a low strength adhesive is used
to bond the padding to the floor. The stiffness of the carpet/padding
combination allows for this low strength attachment to the floor
and allows easy removal. The contractor had used the high strength
adhesive for both the carpet/pad bond as well as the pad/floor
bond, causing the difficulty in removal.
- The end cap of a 190-inch-long paper mill roll broke catastrophically
in the fillet area. A Touchstone investigation found that the
failure was the result of corrosion fatigue where the corrosive
atmosphere in the mill caused pits in the fillet area. Once the
pits became deeper than they were wide, they presented a stress
raiser. Under normal bending forces the crack propagated to failure.
- An insurance adjuster requested that Touchstone investigate
a fire that started in a home pellet stove. While the fire was
small, there was some smoke damage to the house. A Touchstone
scientist visited the home and found that the pellet stove was
not operating at the time of the fire. The fire was found to be
caused by water leaking through an old chimney combined with dust
on the control board. This combination caused a short in the electronics.
The water allowed current to flow from the 110V line input to
a control circuit. The heat from the abnormal electrical flow
carbonized the circuit board making it conductive, and the board
began to smoke.
- An insurance company requested that Touchstone explain what
appeared to be wind damage to a slate roof on a 100-year-old house.
A number of shingles had fallen from the roof during a wind storm.
It was found, however, that the nail holes in the shingles were
still intact. This indicated that the nails holding the shingles
in place had rusted away, allowing the shingles to fall freely
from the roof. Examination of the roof found severe corrosion
of the nails to the point of nonexistence in many cases. The failure
was found to be deterioration of the nails from normal weathering.
- A customer complained to his insurance company about black stains
on the outside of the house. An examination by a Touchstone scientist
found what appeared to be mildew in many places on the outside
of the house. Examination of samples of the black coating by Touchstone's
resident microbiologist found that they were spores from mildew.
Discussions with the owner found that the problem appeared shortly
after the house was painted. It was also learned that recent remodeling
also included the addition of a whole-house air conditioner, which,
because there was no insulation in the exterior walls, caused
all the exterior walls to become relatively cool. This caused
the exterior wall to remain moist for a long enough period of
time to allow the mildew to grow on the outside of the house.
The key to this answer was taken from the observation that the
dark stain pattern on the outside of the house followed the pattern
of the space between the studs inside the walls, the studs providing
just enough insulation for the area outside them them to stay
a little drier, and the space between them to stay wet.
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